1. Field of the Invention
This invention relates to apparatus and processes for the manufacture of steel from iron carbide in two tightly-coupled stages comprising a first stage reactor in which an iron-carbon alloy of intermediate carbon content is produced by combusting energy-rich gases, including such gases from a second stage reactor, in addition to energy-rich gas generated in the first reactor, and such iron-carbon alloy is used as feed to the second stage reactor in which steel of a final desired carbon content is produced.
2. Description of Related Prior Art
Stellung et al. U.S. Pat. No. 2,780,537 discloses a method of producing iron carbide and states that the product may be oxidized to iron in steel furnaces of known construction.
Kalling et al. U.S. Pat. No. 2,978,318 discloses continuous feeding of powdered material containing iron carbide into an inclined, rotary "Kaldo" type furnace for producing steel. The patent teaches that this feed material provides all the heat required for the reaction and does not introduce sulfur into the process.
Raquin et al. U.S. Pat. No. 3,486,882 teaches a process for the continuous production of steel that includes continuously introducing prereduced steel-forming material into a molten bath contained in a refining vessel and simultaneously introducing thermogenic material and gaseous oxygen into the vessel. The prereduced steel-forming material is iron ore which has been reduced between 40 and 100%. This material is introduced into the vessel at an elevated temperature and may contain carbon or other thermogenic elements in sufficient amount to provide necessary heat requirements for the process.
Rouanet U.S. Pat. No. 3,527,598 teaches carrying out a continuous steelmaking process in a reactor using carburized and non-carburized prereduced pellets. The total carbon content of the carburized and non-carburized pellets is such that the reaction with oxygen furnishes all of the heat required for carrying out the process.
Stephens Reissue U.S. Pat. No. 32,247 teaches a process for the production of iron carbide from iron ore by utilizing a fluid bed process. The iron carbide product then is fed into a steelmaking furnace, such as a basic oxygen furnace or an electric furnace, for the production of steel.
While the original Stephens patent, U.S. Pat. No. 4,053,301, describes the furnace as either a basic oxygen furnace or an electric furnace, the above Stephens reissue patent broadly claims a "steelmaking furnace." The Stephens reissue patent prosecution history also states that the type of furnace used in the steelmaking process of the invention is irrelevant to the primary novelty of the patent. Thus, the Stephens reissue patent prosecution history teaches that the Stephens process is not limited to a particular steelmaking furnace, but can include other prior art steelmaking furnaces such as, for example, a reactor vessel.
Additionally, the Stephens reissue patent teaches, at col. 2, lines 20-22, that the formation of iron carbide and its subsequent conversion to steel can be "one continuous operation." Stephens teaches, at col. 4, lines 16-21, that, when the hot iron carbide is added directly to the furnace, the process is "continuous and auto-thermal." Stephens also teaches that the off-gas from the furnace, which contains about 90% carbon monoxide, may be collected and combusted with oxygen to produce heat.
The concept of continuous production of steel, e.g. from iron ore, has been discussed by Queneau in "The QSL Reactor for Lead and its Prospects for Ni, Cu, Fe," Journal of Metals, December, 1989, pages 30-35, and also by Worner: WORCRA (Continuous) Steelmaking, Open Hearth Proceedings, 1969, pages 57-63, and Proceedings of the Savard/Lee International Symposium on Bath Smelting, Minerals, Metals & Materials Society, 1992, pages 83-101.
The Queneau or Queneau-Schuhmann process for continuous production of steel is similar in nature to the so-called "QSL" process for production of non-ferrous metals, e.g. lead and nickel. See, e.g. U.S. Pat. Nos. 3,941,587, 3,988,148 and 4,085,923 and the above-mentioned Journal of Metals article. This latter publication discloses an enclosed reactor vessel for direct and continuous production of steel from iron oxide ores. The QSL reactor is an enclosed system that is capable of limiting the ingress and egress of atmospheric gases and gaseous reaction products.
The WORCRA process and similar processes, such as that described by Rudziki et al. in Open Hearth Proceedings, 1969, pages 48-56, used top lance blowing of oxygen or combined top and bottom oxygen blowing to burn CO generated on top of the melt to generate additional heat for the process. Rudziki's process is used to decarburize liquid pig iron saturated with carbon.
In the so-called "IRSID" process, described by A. Berthet et al. at the International Conference of the Science and Technology of Iron and Steel, Tokyo, September, 1970, page 60 and following, hot metal, such as pig iron, is continuously charged into a reactor into which oxygen is top-blown onto a metal bath, causing formation of a slag/metal/gas emulsion wherein very rapid refining of the metal occurs. The refined steel then moves to a decanter vessel for slag/metal separation and tapping. Carbon content of the feed metal is 4-5% and there is no gradient of carbon level from the entry to the exit end of the reactor. This process also is described in French Patent No. 2,244,822.
Geiger U.S. Pat. No. 5,139,568 discloses a reactor vessel that receives solid mineral material feed. The mineral feed enters a molten metal bath that consists of a lower, denser iron-carbon alloy or metal layer and an upper, lighter slag layer (col. 6, lines 35-37). Oxygen is injected into the molten metal through submerged nozzles and reacts with carbon from the iron carbide to generate carbon monoxide. The carbon monoxide enters a vapor space above the molten bath (col. 6, lines 51-55), where it reacts with oxygen that is injected into the vapor space. The heat from the combustion of carbon monoxide in the vapor space is said to provide about 100% of the heat energy required for the continuation of the reaction in the reactor (col. 7, lines 11-22).
In the structure taught in the Geiger '568 patent, the amount of oxygen injected into the molten metal through the bottom of the reactor is varied along the length of the reactor (col. 9, lines 63-68). In this manner, a carbon content gradient is formed along the length of the reactor and a low carbon alloy is formed for removal at the removal end. Without forming a carbon content gradient, an iron-carbon alloy of sufficiently low carbon is not formed at the removal end of the reactor.
Additionally, in the reactor described in the Geiger '568 patent, the carbon monoxide reaction product passes into the vapor space and oxygen is injected into the vapor space for combustion with the carbon monoxide. The '568 patent teaches that the combustion of carbon monoxide occurs with the oxygen injected into the vapor space. Oxygen from the molten bath is a "highly unlikely" source of oxygen for combustion of the carbon monoxide. Significant amounts of carbon monoxide and oxygen must react in the vapor space to form sufficient heat to make the process self-sustaining or autothermal. Accordingly, all or substantially all of the carbon monoxide reaction product must enter the vapor space and be combusted there in order to generate sufficient heat to further drive the reaction and allow a self-sustaining or autothermal process.
Although the overall heat balance of the Geiger reaction may be substantially correct for his purpose, the problem with the single vessel and accompanying need for a carbon concentration gradient, is that the heat balance does not reflect where in the process there are energy deficits and energy excesses, and how to control and recover the energy release from the combustion of carbon monoxide to carbon dioxide--which is necessary to achieve the provision of energy at the location within the reactor where it is needed.
The Geiger '568 patent recognizes that, in order to operate such a single reactor continuously to achieve the desired low carbon content, a carbon concentration gradient must be maintained from the iron carbide feed end to the tap end of the reactor. The patent teaches that, for the process to be thermally autogeneous, oxygen must be introduced into the vapor space of the reactor to combust the CO generated in the molten metal bath, producing heat and CO.sub.2. The heat so generated is envisaged as being substantially transferred to the molten metal bath and this is a necessary condition for maintaining a thermally autogenous process. However, that disclosure shows serious deficiencies. In the iron carbide feed end of the reactor, the predominant chemical reactions are endothermic, hence external heat is required to keep those reactions going. In the same region, gas evolution volume is high, resulting in a high rate of turbulent diffusion in the metal bath, leading to a well-mixed reaction region. In the remaining portion of the reactor, the predominant chemical reaction is exothermic (decarburization) and is accompanied by the generation of carbon monoxide providing an energy-rich fuel when combusted to carbon dioxide. Due to the elongated geometry of the Geiger reactor, a significant amount of carbon monoxide will be released into the vapor space at locations not in the vicinity of the energy-deficient region where iron carbide is fed into the reactor. Hence, the greater part of the energy released by combustion of carbon monoxide at locations distant from the iron carbide feed region will not reach that region because the radiation heat transfer view factor between that region and the remaining surfaces of the reactor is low. (The radiation heat transfer view factor is used in the art of heat transfer to characterize the effectiveness of radiative heat transfer between surfaces and between gases and surfaces.) Therefore, and from the standpoint of utilizing the greater part of the carbon monoxide energy in the energy-deficient iron carbide feed region, in the method disclosed by Geiger it is not possible to achieve thermal autogeneity. Another problem with the Geiger patent is in respect to the efficiency of utilization of carbon monoxide combustion energy stemming from carbon monoxide generated directly within the iron carbide feed region of the reactor. Since, in the Geiger process, all of this carbon monoxide is combusted in the vapor space of the reactor, the heat of combustion released will tend to be transferred equally well to both the bath surface and the dome refractory walls of the reactor forming the vapor space. In this way, the reactor dome refractory walls will get very hot which, in practice, would require provisions for water cooling. But, since the dome walls contain more surface area than the surface of the molten bath, significant heat losses to the dome walls of the reactor would be experienced. Thus, the amount of energy reaching the bath from the combustion of directly generated carbon monoxide will be only a fraction of the total energy generated. This additional factor further illustrates the point that the process as described in the Geiger patent falls short of being autogenous.
Sohn et al., in Proceedings of the Savard/Lee International Symposium on Bath Smelting, Minerals, Metals & Materials Society, 1992, pages 377-412, provide information concerning relationships between dimensions of a bottom-blown continuous refining reactor to minimize backmixing.